The reason on why most people use BITCOIN as their currency for gambling is due to its value that no other altcoin can match.
Assuming that we were to use a currency entirely dedicated to gambling, the next question would be the price of such currency. Remember what determines the price of BITCOIN is the number of people who uses the currency. Basically, the law of supply and demand governs its price on the market.
Going back, if there exist such currency for gambling, its price would start at rock-bottom and its supply would be relatively huge. This would mean that it's actually priceless until people use it, which I doubt will happen anytime soon.
I don't think it's solely because of the value because bitcoin is not only moving in one direction, it could also pump and dump, it's volatile in short.
The reason I believe is that Bitcoin is the most popular currency that's why more people are using it, and as more and more platform adopting with bitcoin, it will also help to increase its value due to the increase of demand, however, if you are a gambler, your main focus is to win in the game and not the possibility that the money you gamble will rise in value, in fact that's a high risk since you are gambling to win in both price movement and the amount you bet on whatever game you are playing.
For me, I would also love to see more gambling sites accepting a stable coin, at least this will entice gamblers really just play to win the game.